Dyes for sensitizing gelatin silver halide emulsions to be included in photographic elements for exposure to infrared rays are known in the art. They normally include two heterocyclic nuclei linked to each other by a long conjugated methine chain, having, for example, 7, 9 or 11 carbon atoms. Each of the two heterocyclic nuclei are attached at the ends of such a methine chain, particularly through a carbon atom attached to an alkyl substituted nitrogen atom.
One sensitizing dye of this kind can be described as including a resonance chain of the amidinium ion type, as described in Mees and James, "The Theory Of The Photographic Process", third edition, 1966, p. 201. Such a resonance chain or resonance system is made of two nitrogen atoms (alkyl group substituted) at the ends of a conjugated methine chain, one of which is positively charged. Such a conjugated methine chain is partially within the skeleton of said nitrogen-containing heterocyclic nuclei. When speaking in the art, however, about conjugated methine chains between two heterocyclic nuclei, having 7, 9 or 11 carbon atoms, the methine groups of the chain within the heterocyclic nucleus skeleton are not considered in the counting of methine groups (they are normally present as one group, as in benzothiazole and 2-pyridine dyes, or two groups, as in 4-pyridine dyes). The above numbers 7, 9 or 11 thus indicate only the carbon atoms of the conjugated methine chain skeleton which bridges said two heterocyclic nuclei. It is assumed that the point of attachment of the chain to the heterocyclic nucleus is at a carbon atom directly or indirectly attached to the nitrogen atom of the amidinium ion resonance chain, indirectly meaning attached only through a sequence of two conjugated carbon atoms. As known, the nature of the atoms, which form the heterocyclic nucleus, as well as the nature of the substituents attached thereto and the alkyl groups attached to the nitrogen atom itself, and the length and nature of the methine chain between said heterocyclic nuclei have affects on such resonance and on various characteristics of the resulting dyes.
It is known, for example, that heavier heterocyclic nuclei as well as longer chains cause a bathochromic effect on sensitization peaks. It is further known that such long chains may be associated with lack of stability unless they are rigidized by substituting the 2nd through 4th or 3rd through 5th methine groups in with carbon atoms respectively to form a 6- or 5-membered carbon ring. Reference can be made to U.S. Pat. No. 2,734,900 describing chains rigidized on a 6-membered ring and to U.S. Pat. Nos. 3,482,978 and 3,758,461 and GB Pat. No. 1,188,784 describing chains rigidized on a five-membered ring.
Such heterocyclic nuclei linked together through said methine chain may be chosen among those 5-, 6- or 7-membered heterocyclics comprised of C, N, S, O and Se ring atoms which may also have fused on benzene or naphthalene rings known in the art such as, for example, thiazole, benzothiazole, [1,2-d]-naphthothiazole, [2,1-d]-naphthothiazole, oxazole, benzoxazole, selenazole, benzoselenazoline, [1,2-d]-naphthoselenazole, [2,1-d]-naphthoselenazole, thiazoline, oxazoline, selenazoline, 2-quinoline, 2-pyridine, 3,3-dialkyl-indolenine, 4-quinoline, 4-pyridine, imidazole and benzimidazole. Various substituents can be attached to each of them in their various reactive positions as known in the art. Such substituents and their position can be chosen to introduce desired variations into the characteristics of the dyes and their sensitization properties, such as stability, solubility and sensitization peak. In spite of the many experiments made in the art, however, the capability of such dyes to sensitize the silver halide emulsions to the infrared rays still remains unsatisfactory.